This invention relates to incoherent nonphaselocked laser arrays and in particular nonphaselocked or optically uncoupled laser arrays having a broad uniform far field with high intensity which are highly adaptable for use as illuminating sources with electro-optic line modulators and electro-optic line printers.
In the prior art, semiconductor lasers and LEDs have been employed as a light source to produce an image on a photosensitive medium such as a xerographic photoreceptor used in a xerograhic printer. In this kind of application, there is a need for uniformity of the intensity of light in the image as well as sufficient intensity of the light emitted. Further, if LEDs have to be utilized, it is necessary to provide a full width array of LEDs, one per picture element or pixel so that a line of light can be formed for discharge of the photoreceptor in an imagewise manner. Usually a plurality of light emitting device arrays are arranged in one or more rows and optical means is positioned between the photosensitive medium and the light source array to focus the light from the array sources to a single line on the surface of the photosensitive medium. The light sources are selectively turned on and off to effect line-by-line exposure of the moving photosensitive medium.
Semiconductor lasers have also been used in the part as such light sources for rotating polygon scanned printers because of their high intensity in a focussed spot. They have not, however, been totally best suited for application in electro-optic line printers due to inadequate power and inadequate uniformity of light intensity. In particular, high-power coherent laser sources possess a far field pattern containing regions of high intensity and of low intensity in the single beam far field, i.e., the far field pattern is not uniform. Such a variation in intensity across the beam output is not desired because the line exposure on the photosensitive medium will not be uniform. For this and other reasons, LEDs have been more favored as a light source for electro-optic line printers because they may be designed to provide a light output of monotonically varying intensity with very short coherence length.
In some cases, LEDs in the past have not provided sufficient output power and intensity to accomplish in an efficient manner the task of exposing a charged an moving photosensitive medium. In addition, an LED is much less efficient than a laser. For this reason, LEDs as light sources for xerographic photoreceptor applications have lacked the output intensity level for good photoreceptor discharge and as a result, semiconductor lasers have been still favored in many cases as the light source for printer applications.
Beside the problem of sufficiency of LED intensity, the maintenance of light output uniformity among a plurality of LEDs as well as a multiple laser source, as alluded to above, is a recognized problem in the art. To insure that the intensity of the broad light emission from the array is uniform across an LED array, elaborate control systems have been designed to provide for light intensity uniformity as exemplified, for example, in U.S. Pat. No. 4,455,562. This patent utilizes a binary weighted duty cycle control to obtain substantial uniformity in the light emitted from each LED in the array of LEDs.
The highest power LEDs have been top emitter types, but they lack power density necessary for most printer applications, i.e., they lack from sufficient light density per aperture size.
A most recent advancement in the printer arts has been the concept of a total internal reflection (TIR) line modulator which is a solid state multigate light valve that may be used to address a photosensitive medium. The light TIR modulator comprises a crystal bar of electro-optic material with an array of interdigital electrodes deposited on one of its major surfaces, which electrodes, when electrically addressed, introduce or induce a periodic electric field into the bulk crystal. Each of the electrodes may be individually addressed by an electronic signal forming a signal pattern across the array. A broad or wide sheetlike beam of high intensity light is required for the line modulator. The beam is introduced into the crystal at an angle incident to the plane of the major surface incorporating the electrodes. An example of the line TIR modulator is disclosed in U.S. Pat. No. 4,281,904 to Robert A. Sprague et al.
To carry out the exposure process of the photosensitive medium, a sheetlike beam of light is transmitted through the electro-optic element of the TIR line modulation at a slight angle relative to the optical axis of the light to cause total internal reflection at the internal surface incorporating the electrode array. Successive sets of digital bits or analog samples, representing respective collections of picture elements or pixels for successive lines of an image, are sequentially applied to the electrode array. Localized electric bulk or fringe fields developed in the crystal in proximity to the TIR incidence of light modulate the light and change the phase front of the sheetlike light beam in imagewise configuration onto the charged photosensitive medium. Examples and teachings relative to electro-optic line printer applications may be found in U.S. Pat. Nos. 4,367,925; 4,369,457; 4,370,029; 4,437,106; 4,450,459; 4,480,899 and 4,483,596.
More recently, a super luminescent LED side-facet source has been developed for electro-optic line modulation and line printers which is characterized by having high output intensity and a uniform far field emission and optical means to collimate the far field emission in the tangential direction and focus the near field in the sagittal direction onto the modulator. The optical means comprises a first lens system to collect the light emitted from the LED source in both the tangential and sagittal directions and a second toric lens to collimate the light into a sheetlike beam in the tangential direction and to focus the light in the sagittal direction to a line image at the modulator. Imaging means is optically aligned between the modulator and the recording medium for imaging the modulator onto the recording medium of a line printer. In this connection, see U.S. patent application Ser. No. 719,595 filed Apr. 3, 1985, now U.S. Pat. No. 4,638,334 and U.S. patent application Ser. No. 874,982, filed June 16, 1986, a divisional application thereof.
Such an LED has nearly ideal characteristics as an optical source in printing applications utilizing multigate or electro-optic modulators because its radiation pattern is broad and varies monotonically in a predictable way without the sharp or irregular structure encountered with a diode laser array. In addition, the optical spectrum of the LED is sufficiently broad that optical interference effects are negligible. However, an LED inherently has a lower overall efficiency, e.g., conversion efficiency, than a diode laser since LED light is emitted in many different directions and also this requires that the LED of comparable output power operates at higher temperatures and higher input power than the diode laser source.
Thus, an optical source with the incoherence of an LED but the efficiency of a diode laser is desirable for line modulator and printer applications.